1. Field of the Invention
The present invention relates to a thermally assisted magnetic head which writes signals by a thermally assisted magnetic recording scheme, a head gimbal assembly (HGA) equipped with the thermally assisted magnetic head, and a hard disk drive equipped with the HGA.
2. Related Background Art
As hard disk drives have been increasing their recording density, thin-film magnetic heads have been required to further improve their performances. As the thin-film magnetic heads, those of composite type having a structure in which a magnetism detecting device such as magnetoresistive (MR) device and a magnetic recording device such as electromagnetic coil device are laminated have been in wide use, while these devices read/write data signals from/onto magnetic disks which are magnetic recording media.
In general, a magnetic recording medium is a sort of discontinuous body in which magnetic fine particles are assembled, while each magnetic fine particle has a single-domain structure. Here, one recording bit is constituted by a plurality of magnetic fine particles. Therefore, for enhancing the recording density, it is necessary to make the magnetic fine particles smaller, so as to reduce irregularities at boundaries of recording bits. When the magnetic fine particles are made smaller, however, their volume decreases, so that the thermal stability in magnetization may deteriorate, thereby causing a problem.
An index of the thermal stability in magnetization is given by KUV/kBT. Here, KU is the magnetic anisotropy energy, V is the volume of one magnetic fine particle, kB is the Boltzmann constant, and T is the absolute temperature. Making the magnetic fine particles smaller just reduces V, thereby lowering KUV/kBT by itself, which worsens the thermal stability. Though KU may be made greater at the same time as measures against this problem, the increase in KU enhances the coercivity of the recording medium. On the other hand, the writing magnetic field intensity caused by a magnetic head is substantially determined by the saturated magnetic flux density of a soft magnetic material constituting a magnetic pole within the head. Therefore, no writing can be made if the coercivity exceeds a permissible value determined by the limit of writing magnetic field intensity.
Proposed as a method for solving such a problem in thermal stability of magnetization is a so-called thermally assisted magnetic recording scheme which imparts heat to a part of a recording medium by irradiation with light from a light source at the time of applying a writing magnetic field, while using a magnetic material having a large value of KU, so as to effect writing with lowered coercivity by a magnetic recording device (see, for example, Japanese Patent Application Laid-Open No. 10-162444, No. 2001-255254, No. 2004-158067, No. 2006-185548, IEEE Trans. Magn. Vol. 41, pp. 2817-2821 (2005)).